1. Field of the Invention
This invention relates to implantable pacemakers and more particularly to an improved cardiac interface which includes a new type of sense amplifier.
2. Description of the Prior Art
The cardiovascular system provides oxygenated blood to various structures of the body. The body's demand for oxygenated blood is reflected by the rate at which the sinus node of the heart beats. The electrical signal generated by the sinus node causes the atria or upper chambers of the heart to contract, forcing blood into the lower chambers or ventricles of the heart. After a brief delay, the lower chambers of the heart contract forcing the blood through out the body. The contraction of the ventricles proceeds in an organized fashion which is reflected by the passage of a depolarization wavefront through the syncytium of the heart muscle.
Various disease mechanisms cause conduction disturbances which interfere with the natural conduction system of the heart. A variety of implantable medical devices have been developed to treat these abnormalities. The pacemaker is an example of one such implantable medical device which supplies therapeutic stimulation to the heart to compensate for these conduction defects. Pacemakers as well as other implantable medical devices require interface circuitry which is used to interconnect cardiac sensing circuits and cardiac stimulating circuits to the heart.
The first pacemakers paced the heart at a metronomic rate independent of the hearts underlying rhythm. Such pacemakers are typified by U.S. Pat. No. 3,057,356 to Greatbatch. One problem with such pacemakers is that they may compete with the heart's underlying rhythm and provoke lethal arrhythmias.
The demand pacer was introduced to overcome this defect. This form of pacer contains circuitry to detect the depolarization of the cardiac tissue. The circuitry for performing this function is referred to as a sense amplifier in this art. The function of the sense amplifier is to generate a sense event signal which is used by the escape interval timer of the pacer to resynchronize the pacer to the heart's rhythm. In operation the pacer escape interval timer is set to a nominal stimulation rate which reflects the lowest permissible heart rate. If the underlying heart rate is above this standby rate, the pacer detects the cardiac depolarization and prevents the delivery of pacing stimuli. This form of pacer is now classified as a VVI mode pacer and is taught to the art by U.S. Pat. No. 3,345,990 to B. Berkovitz. The efficacy and safety of this pacing modality requires a reliable sensor of heart activity.
The sense amplifier itself has undergone steady development and refinement as reflected by the teachings of U.S. Pat. No. 4,275,737 to D. Thompson; U.S. Pat. No. 4,379,459 to M. Stein; and U.S. Pat. No. 4,644,931 to R. Beck. However throughout this development, the underlying design philosophy has remained the same.
In the prior art, it has been common to use very high impedance amplifiers which do not substantially load the signal source to amplify the voltage difference which is generated across the electrode pair by the passage of a cardiac depolarization. This prior approach suffers from a variety of problems which relate to the use of high gain factors necessitated by the low level signal generated by the heart. These prior art techniques rely on pass band filters and forms of time domain filtering to achieve acceptable results.
This prior art sense amplifier architecture is easily saturated by the pacing pulse. For this reason, the pacer output stage may be decoupled or isolated from the pacer sense amplifier to help prevent erroneous detection of the pacing artifact. In general, contemporary pacers adopt "blanking", "refractory periods" and "fast recharge" structures to minimize the effects of the interaction between the output stage and the sense amplifier.
It is also possible to minimize interaction between the sensing and pacing functions by dedicating separate electrodes to the pacing and sensing functions. However, lead size and pacer can feedthrough considerations usually dictate lead systems in which electrodes are used for both functions.